The present invention relates generally to a system and method for calibrating a digital color printer, such as an electrophotographic or ink-jet color printer, and more particularly relates to the use of digital halftone-algorithm independent techniques for characterizing the digital color printer.
Digital halftoning is a process of displaying continuous tone images in an output device such as a printer. Most conventional printers, such as ink-jet printers and electrophotographic xe2x80x9claserxe2x80x9d printers, ultimately operate in a binary mode, which means that a printed dot is either present or absent at a specified location on a two-dimensional medium. Thus, prior to printing an image with the image output device, such as a digital color printer or digital color press, continuous-tone color images are halftoned into binary color input signals. The binary color input signals transmitted to a color printer describe the binary modes of colors for each printed dot. Typically in a digital color printer, these colors are cyan (C), magenta (M), and yellow (Y) or CMY plus black (K). For example, in a CMY digital color printer, a color dot is printed based upon the binary modes of CMY. Selected combinations of CMY may produce other colors, including red (R), green (G), blue (B) and black (K), or white (W) representing the absence of color. Thus, there are eight possible combinations of CMYRGBKW for each printed color dot.
Most color printers used today are calibrated prior to use. Calibration refers to the process of setting the threshold values of the color input signals transmitted to a color printer so that a given input color is well represented by the printed image. For example, if the input color signal is greater than the threshold value, a corresponding dot is printed. Otherwise, a dot is not printed. This calibration procedure is conducted every time the printing environment of the digital color printer is changed, for example, when the printer medium is changed. Also, in order to ensure a consistently high quality printer output, the calibration procedure can be conducted regularly, such as daily or weekly. In the calibration process, it is necessary to characterize the color printer to provide a set of data which is used to correct the threshold values of the color input signals. Thus, a series of color patches are printed on the medium with a color printer and an optical characteristic of the color patches is measured in order to characterize the color printer.
Currently almost all color calibration methods are halftone-algorithm dependent. Specifically, calibration conducted using one halftone algorithm can not be applied to another halftone algorithm without losing color accuracy. Halftone-algorithm independent calibration techniques for grey scale and color printers have been described in the art, but have not achieved high quality color reproduction. Examples of such techniques are described in the below cited patents and publications, the contents of which are hereby incorporated by reference.
U.S. Pat. No. 5,469,267, by the inventor hereof, describes a 2-by-2 halftone correction system for correcting digital image signals corresponding to a continuous tone image for the effects of printed dot overlap generated by a particular digital printer. The dot overlap correction is achieved by superimposing a virtual screen on the printer-generated dot patterns, such that the printer dots are entered at the orthogonal intersections of the lines defining openings in the screen. This centering approach allows for the determination of printed dot overlap by a 2-by-2 matrix, so that only seven test patterns are required for characterization of the printer and for dot overlap correction of halftone prints produced by the printer.
U.S. Pat. No. 5,748,330, also by the inventor hereof, describes a method of calibrating a digital printer using component test patches and the Yule-Nielsen equation. The technique for calibrating the hardware and software of a digital printer relies on making seven component test patches which completely characterize the printing system, and then measuring the actual reflectance of the seven test patches. The measured reflectances are then converted, by the Yule-Nielsen equation, to values representative of the ink area coverage characteristic of each component test patch, since ink area coverage is the parameter that can be directly controlled by the digital printer. This conversion step takes into account the non-linear relationship between the reflectance of a halftone area and the amount of ink area coverage on the halftone area.
U.S. Pat. No. 5,854,882 describes a halftone correction system for producing dot overlap corrected halftone images at a digital printer. The system is provided for producing dot overlap corrected halftone color images on a digital color printer. The system first calibrates the digital color printer with a set of color test patterns to provide halftone correction information. Next, multi-level digital color image signals representing a continuous-tone color image are received and stored. The system then halftones the multi-level digital color image signals to provide an overlap corrected halftone color image responsive to the halftone correction information at the digital color printer. The system may halftone the continuous-tone color image by either color error diffusion or ordered dithering techniques.
In addition to the U.S. patents described above, the article xe2x80x9cAlgorithm-Independent Color Calibration for Digital Halftoning,xe2x80x9d (Proceedings of ISandT/SID, 4th Color Imaging Conference, 1996), of which the present inventor is the author, describes a method based on measuring 2-by-2 pixel patterns which provide halftone algorithm independent color calibration for digital halftoning where the binary CMY(K) color signals corresponding to an image can be mapped into CIE XYZ color space at the printer resolution level. Therefore, any binary CMY(K) color images can be described as continuous tone images in standard color spaces. The teachings of this article are incorporated by reference.
The difference between the measured and the predicted optical characteristic of a test patch is an objective measurement of color difference in predicting output colors in a color printer. As shown in the article described above, the average difference between the measured and the predicted optical characteristics of 125 test patches by the 2-by-2 centering technique described above is around 5.0, and the maximum difference between the measured and the predicted optical characteristics of test patches among 125 samples is around 11.0. The above-described values of the average and the maximum differences between the measured and the predicted optical characteristics of the test patches are generally unsatisfactory and typically result in poorly reproduced color images. There is, thus, a need in the art to improve the average and the maximum differences in values between the measured and the predicted optical characteristic of test patches in order to more accurately predict the optical characteristics of test patches printed by the digital printer.
The present invention provides a method and system for calibration of a digital color printer to accurately predict the optical characteristic of color patches printed in the color printer. The present invention is based on measuring 2-by-2 pixel patterns to provide a halftone-algorithm independent characterization of a digital color printer, where all printed dots are located at intersections of a superimposable, virtual grid or coordinate system. The present invention provides a further manipulation of the 2-by-2 patterns to accurately predict the optical characteristic of selected test patches printed in a digital color printer, which effectively considers the optical scattering in a printing medium. As a result, the present invention provides for a printer calibration technique that more accurately predicts colors of halftone prints.
According to one aspect of the present invention, there is provided a method for calibrating a digital color printer. In the method, a plurality of elementary halftone color patches are created based on a 2-by-2 centering technique to provide a halftone-algorithm independent characterization of a digital color printer. In the 2-by-2 technique, each of the elementary halftone color patches represents a pattern of dots relative to a virtual, superimposable grid. An optical characteristic, such as reflectance, of the elementary halftone color patches printed on a medium by the printer is measured. After the optical characteristic of the elementary halftone color patches is measured, the measured characteristic is compensated for by a selected value associated with at least the medium. As a result, an optical characteristic of one or more arbitrary halftone color patches created according to a halftone algorithm can be predicted based on the compensated optical characteristics of the elementary halftone color patches.
According to another aspect of the present invention, there is provided a system for calibrating a digital color printer. The system operates based on a plurality of elementary halftone color patches created according to 2-by-2 patterns which provide a halftone-algorithm independent characterization of a digital color printer. In 2-by-2 patterns, each of the elementary halftone color patches represents a pattern of dots relative to a virtual, superimposable grid. The optical characteristic of elementary halftone color patches printed on a medium is measured and stored in look up tables. The optical characteristic of the elementary halftone color patches stored in the look up tables is converted by a compensation value that considers the optical characteristics of at least the medium. Consequently, an optical characteristic of arbitrary halftone color patches created according to a halftone algorithm can be calculated based on the compensated optical characteristic of the elementary halftone color patches.
According to the present invention, the accuracy in predicting the optical characteristic of arbitrary halftone color patches is improved by considering the optical scattering in the medium. The accuracy can be defined as the average of the difference between the measured and the predicted optical characteristic of test patches and the maximum difference between the measured and the predicted optical characteristic of test patches. Hence, according to the present invention, the accuracy in predicting the color output of a particular color printer has been improved from 4.9 and 12.4 to 3.0 and 5.8, respectively, by considering the optical scattering of the medium and by providing a compensated value. As a result, the calibration of the color printer can be performed with high efficiency and can achieve accurate full-scale color reproduction of input images because of the improved accuracy in predicting the optical characteristic of the halftone color output of the printer.